(1) Field of the Invention
The invention relates to a device and method for sensing the curvature of a towed array. The device of the present invention, in combination with other sensors, may be used to determine the shape of a towed hydrophone array.
(2) Description of the Prior Art
Optical fibers have been used in a variety of sensors. For example, U.S. Pat. Nos. 4,654,520; 4,812,645; and 4,927,232, all to Griffiths, illustrate structural monitoring systems which have an optical fiber securely and continuously fastened to a structure such as a pipeline, offshore platform, bridge, building, or a dam or to a natural object. A light signal is passed into one end of the optical fiber. Any physical movement of the structure, or sectional movements along the optical fiber path, such as deflection, bending, displacement, or fracture of the structure affects the optical fiber. As a consequence, detectable changes occur in the electro-optical signature or in the light signal transmission.
U.S. Pat. No. 5,321,257 to Danisch illustrates a fiber optic bending and positioning sensor which is composed of a fiber optic or light wave guide for attachment to the member which is to be bent or displaced. Light is injected at one end and detected at the other end. Bending of the fiber results in light loss through a surface strip or band, along one side of the fiber, this loss being detected. The loss of light detection is used to produce indication of bending or displacement. Two or more light guides can be oriented to give indication of the direction of bending or displacement.
One of the deficiencies of these systems however is that the optical fiber(s) used in the sensor is/are attached directly to the structure whose behavior is being observed.
Some towed hydrophone arrays require precise determination of their shape in the water. This has been done in the past with gimbaled heading sensors. Such sensors are quite expensive. They are unsuitable in today""s environment where one needs to reduce cost in a towed array. It is also desirable in modern towed arrays to provide shape sensing that is compatible with optical hydrophones and that is relatively inexpensive to perform.
An alternative way to determine array shape is by curvature sensors and either roll or twist sensors. It has been proposed to use fiber optic sensors to sense curvature. Such sensors embed optical fibers containing Bragg gratings in the hose wall of the towed array. The Bragg gratings sense the strain in the hose wall when the array is bent and the differential strain from the outside to the inside of the bend permits calculation of the curvature. However, the strain seen in the hose wall as the array passes over small diameter handling sheaves can exceed the survival strain of an optical fiber. It has been suggested to reduce the strain seen by winding at a pitch angle, but that approach is awkward. It has also been suggested to reduce strain by minimizing the distance each fiber is placed from the centerline of the array. The disadvantage of all these mounting schemes is that while limiting the maximum strain seen, these schemes also limit the strain sensitivity achievable. One may define a total dynamic range of curvature from the maximum curvature of the handling system sheaves to the minimum curvature associated with the ultimate array position accuracy desired. This range may be 50 dB. Meanwhile, the sensor system actually only has to operate in towing conditions where the range of curvatures seen may be less than 30 dB.
Thus, there remains a need for a system which senses the curvature of a towed array as well as the shape of the towed array.
Accordingly, it is an object of the present invention to provide a sensor system which senses the curvature of a towed hydrophone array.
It is a further object of the present invention to provide a sensor system as above which can in combination with other sensors may be used to determine the shape of a towed hydrophone array.
It is yet a further object of the present invention to provide a sensor system as above which achieves high strain sensitivity while limiting the maximum strain seen by the optical fiber(s) therein so that only the operational dynamic range is required of the optical fiber(s).
The foregoing objects are attained by the curvature sensor of the present invention.
In accordance with the present invention, a curvature sensor is provided. The curvature sensor broadly comprises a bend member which bends as the array into which it is incorporated bends, at least one optical fiber within the bend member, and at least one detection device embedded within the at least one optical fiber to detect a change in strain in the at least one optical fiber.
A system for detecting the curvature in a towed hydrophone array comprises at least two of said curvature sensors positioned along the length of the array.
A system for also detecting the shape of the towed array includes a roll sensor positioned adjacent each of the curvature sensors.